bussell.] VALLEYS. 19 
lying formation is fine silt and volcanic dust, essentially the same as 
the material just referred to as having been deposited in Lakes Idaho 
and Payette. Conspicuous examples of such deposits occur in the 
valley of the Owyhee where Jordan Creek enters it, about the eastern 
and southern margins of Malheur and Harney lakes, and in many 
other localities. The picturesque badland sculpturing of the lacus- 
tral deposits in the valley of the Owyhee is represented on PL XVIII. 
In general, as may be gathered from the statements just made, the 
valleys of the region examined are floored with the deposits of Terti- 
ary lakes and by modern alluvium washed in from the adjacent 
uplands. Another process of filling is through the action of the wind. 
Considerable removal and redeposition of fine material is now going 
on, and in certain localities notable accumulations of sand and dust 
can be easily traced to this cause. Still another process of valley 
filling, which results in giving depressions remarkably level and con- 
spicuously persistent surfaces, is by the accumulation of rock frag- 
ments, such as lapilli and volcanic dust blown out of volcanoes during 
explosive eruptions, and the overflow of highly liquid lava from vol- 
canic vents, as will be explained later in describing some of the 
recent volcanoes of the region under consideration. 
The wide distribution of fine alluvium over valley floors by ephem- 
eral streams necessitates a delicate adjustment of climatic and 
topographic conditions such as is seldom seen except in regions of 
small rainfall. Before man disturbed the balance of natural condi- 
tions, many of the valleys of Idaho and Oregon became deeply filled 
with fine, usuall}^ yellowish or nearly white rock debris, and only faint 
stream channels, if any at all, could be distinguished on the surfaces 
of the deposits during the seasons of extreme desiccation. The 
uplands and valleys alike were clothed with an open growth of vege- 
tation, consisting largely of bunch grass. On the uplands and hill- 
sides the grasses serve to bind the soil together, but less completely 
and efficiently than is usually the case in humid regions where a con- 
tinuous sod is present. In the valleys, owing to the absence in many 
instances of stream channels in the deep alluvium, soil moisture was 
held near the surface at least during a considerable part of the dry 
season, and grasses, notably the coarse rye grass, grew luxuriantly. 
This delicate balancing of conditions, a result of a long period of 
adjustment, was seriously disturbed when stock was introduced and 
grazing on the natural pastures was carried on extensively. The 
introduction of large numbers of sheep, especially, has resulted in the 
nearly complete destruction of the bunch grass over vast areas, and 
in consequence the surface run-off from the uplands has been rendered 
more rapid and rillo have been formed where previously the rain 
water soaked into the ground and percolated slowly away. The 
more rapid surface run-off has caused the hillsides to be deeply 
gullied, much of the soil has been swept into the lowlands, and on 
account of the increased strength of the surface streams the debris 
